Hello Melissa Usually, tissues are fixed in 4% PFA (Paraformaldehyde) for 3 to 4 hours at 4 °C. 10% formalin in PBS can also be used. Tissues fixed for long time in formalin may not give good result. However, it depends on what type of tissue you are using. K.R. Vinod, Ph.D. Glenmark Research Centre, New Bombay.